DE69302319T2 - Trickle heat exchanger and air separation device with such a heat exchanger - Google Patents

Description

The present invention relates to a trickle-flow liquid heat exchanger for evaporating a liquid by exchanging heat with a second fluid medium, comprising a parallelepiped body formed by an arrangement of parallel vertical plates which form between them a plurality of flat passages which are divided into a group of evaporation passages and a group of heating passages, each passage comprising a corrugated intermediate piece with vertical surface lines in the area of the heat exchange flow, means for distributing the liquid being provided at the upper end of the exchanger, for distributing the liquid over the entire length of the evaporation passages, and means for introducing the second fluid medium into the heating passages. Such an exchanger is known, for example, from document EP-A-0 130 122. This relates in particular to arrangements for air circulation.

In double column air distillation devices, the liquid oxygen contained in the low pressure column tank is vaporized in heat exchange with the gaseous nitrogen from the top of the medium pressure column. For a given operating pressure of the low pressure column, the temperature difference between the oxygen and the nitrogen, which necessarily results from the structure of the heat exchanger, sets the operating pressure for the medium pressure column. It is therefore desirable to make this temperature difference as low as possible in order to minimize the cost of compressing the air to be treated and injected into the medium pressure column.

In order to achieve this objective by exploiting the particularly advantageous technology of brazed plate heat exchangers, EP-A-0 130 122, in the name of the applicant, has proposed a particularly efficient process for distributing liquid oxygen.

However, current technology has certain limitations, regardless of the distribution method used. These arise from the fact that the gaseous oxygen resulting from evaporation must escape from the exchanger on its own, even though the liquid oxygen is at a pressure only slightly above atmospheric pressure. The pressure loss along the gaseous oxygen flow path must therefore be very low. In all known solutions, this requirement limits the height and, in general, the performance of the exchanger.

The invention aims to make it possible to increase the height of such a heat exchanger or, for a given height, to reduce the pressure loss of the flow of vaporized oxygen. To this end, it has as its subject a heat exchanger of the aforementioned type, characterized in that said distribution means are arranged in chambers which are closed at the upper end and are each located above a heating passage from which they are separated by a horizontal web, in that a horizontal gap extends directly above the web over the entire length of the exchanger and freely connects the lower part of the chamber to an adjacent evaporation passage, and in that the evaporation passages are open at their two ends, top and bottom, over their entire length and comprise at least one corrugated intermediate piece with vertical shell lines along their entire length.

According to special embodiments of the invention

- the evaporation passages in the area of the gap do not have a corrugated intermediate piece;

- the upper surface of the web is inclined laterally towards the gap;

- the means for distributing the liquid comprise, on the one hand, a horizontal barrier extending along the entire length of each chamber at its mid-height, the thickness of this barrier corresponding to the distance adjacent plates and comprises openings for pre-distribution of the liquid, and on the other hand, under this barrier, a lining for fine distribution of the liquid over the entire horizontal length of the chamber;

- the said openings form a horizontal row of equally spaced holes;

- the barrier has on a vertical surface one or more rear recesses which are closed at the bottom and open at the top, and on another vertical surface one or more front recesses which are open at the bottom and closed at the top, and said openings are arranged in a vertical common wall of the front and rear recesses,

- the barrier has a plurality of spaced-apart rear recesses and a plurality of spaced-apart front recesses;

- the recesses on the front have a conical shape that widens downwards;

- the lining is a corrugated intermediate piece with horizontal shell lines, the flanks of which are provided with incisions;

- the lining is spaced from the upper surface of the web;

- the exchanger comprises a lateral liquid inlet chamber at said chamber, the low point of this chamber being below the low point of the inlet window of the chamber.

The invention also relates to an air separation device by distillation, comprising a first distillation column operated at a relatively high pressure, a second distillation column operated at a relatively low pressure, and a heat exchanger which allows liquid oxygen from the container of the second column to be brought into heat exchange with gaseous nitrogen from the top of the first column, characterized in that the heat exchanger is designed as described above and in that the device comprises supply means for supplying liquid oxygen to said means for distributing the liquid and means for supplying gaseous nitrogen to the heating passages.

The embodiments of the invention will now be described with reference to the accompanying drawing. This shows:

Figure 1: a partial diagram of an air distillation device according to the invention;

Figure 2: a vertical section of area II of Figure 1 on a larger scale, along the line II-II in Figure 4;

Figure 3: a partial plan view in the direction of arrow III of Figure 2;

Figure 4: a section along the line IV-IV in Figure 2; and

Figure 5: an analogous representation of a further embodiment.

Figure 1 shows a possible embodiment of an oxygen-nitrogen heat exchanger in a double column air distillation device. This device comprises a medium pressure column 1, in the lower part of which the air to be treated is fed under an absolute pressure of the order of 6 bar. The oxygen-enriched liquid collected in the container of column 1 is fed in reflux to the middle height of a second column (not shown), namely the said low pressure column, which is slightly above atmospheric pressure. The gaseous nitrogen located in the top of column 1 is brought into indirect heat exchange with the liquid oxygen collected in the low pressure column vessel; the resulting condensed nitrogen serves as reflux to column 1 and the low pressure column, while the resulting vaporized oxygen is returned to the bottom of the low pressure column.

The two distillation columns can be equipped with linings, which also contributes to energy gain by lowering the working pressure of the device, which corresponds to that of column 1.

The heat exchange between the oxygen and the nitrogen takes place in an exchanger 2 which is arranged above the column 1 while the low-pressure column is arranged next to the latter.

The exchanger 2 consists of a sealed container 3 which contains, over most of its height, an arrangement of parallel rectangular plates 4 made of aluminium, with a length of about 1 to 1.5 m and a height of about 3 to 7 m, between which the corrugations, also made of aluminium, are fixed by soldering.

A chamber at a pressure slightly higher than that of the low-pressure column (for example, approximately 1.4 bar), located at the level of the upper end of the plates 4, opposite one of their vertical walls, contains a liquid oxygen bath 5 fed by trickle via a pipe 6 coming from the tank of the low-pressure column and equipped with a pump (not shown). The latter can be controlled by means of a level regulator in the bath 5 or, alternatively, by a flow regulator. At the top of the exchanger 2, the tank 3 forms a dome 7 containing the bath 5. From this dome, to the bottom of the low-pressure column, there is a return pipe 8 for the vaporized oxygen coming from the bath 5 due to the heat supplied at the level of the pump and the pipe system and from part of the oxygen vaporized in the exchanger 2.

The arrangement of plates 4 is fed at its upper part with gaseous nitrogen at 6 bar from a horizontal inlet chamber 9 arranged under the bath 5 and connected to the head of the medium-pressure column via a pipe 10. The evacuation of the condensed nitrogen takes place at the bottom of the plates 4 by means of a horizontal collecting chamber 11 which is connected by a pipe 12 to a collecting trough 13 arranged at the head of the column 1. A pipe 14 for the evacuation of small non-condensable gases extends into the chamber 11.

A pipe 15 connects the sump of the low-pressure column with the space located in the vessel 3 under the plates 4. This pipe penetrates vertically through the bottom of the vessel 3 into this space, and a conical deflector 16 is mounted on its upper end. A pipe 17 also extends from the bottom of the space 3, which serves to return the excess liquid oxygen to the vessel of the low-pressure column.

The structure of the active part of the exchanger 2, i.e. the plate arrangement 4, will now be described with reference to Figures 2 to 4.

In this area, the exchanger has a parallelepiped shape and the container 3 is delimited by the walls of the plates 4 and by intermediate webs, except at the inlet and outlet points of the fluids. The plates 4 form a plurality of passages which are alternatively provided for the outflow of oxygen (passages 18) or the outflow of nitrogen (passages 19). Over most of their height, the passages 18 and 19 each contain a corrugated intermediate piece 20 which consists of a perforated corrugated aluminum sheet with vertical casing lines.

The corrugations 20 of the nitrogen passages end at the top and bottom in front of the corrugations 20 of the oxygen passages. In the lower area of the plates 4, these corrugations of the passages 20 are separated by oblique corrugations for collecting the nitrogen (not shown) leading to the inlet of the collection chamber 11. At their upper end, the same corrugations 20 are extended by oblique corrugations 21 for the distribution of the nitrogen, which open into a side window 21A of the exchanger at the outlet of the inlet chamber 9. Above the corrugations 21, the nitrogen passages 19 are closed by horizontal webs 22. Other horizontal webs (not shown) close the lower end of the nitrogen passages below the nitrogen collection zones. Above the webs 22, each nitrogen passage is extended by a chamber 23 for the distribution of the liquid oxygen, which is closed at the upper end of the exchanger by a horizontal web 24. The chamber 23 comprises, from top to bottom: an oblique corrugated intermediate piece 25 (or alternatively a perforated corrugation with horizontal generatrix) for coarse distribution of the liquid oxygen over the entire length of the chamber, this corrugation opening laterally into the bath 5 (Figure 2) via a side window 26 of the exchanger; a perforated barrier 27 for pre-distribution of the liquid oxygen; and a lining 28 for fine distribution of the liquid oxygen. A free space 29 is located between this lining and the upper surface of the web 22.

The perforated barrier 27 is made from a parallelepiped blank, the thickness being equal to the spacing between the plates 4 of approximately 5 to 15 mm and the length being equal to that of these plates. On one of these large sides there is a series of rear recesses (with reference to Figure 2) 30, U-shaped and open at the top, and on the other side there is a series of front recesses 31, approximately semicircular and open at the bottom. Each recess 31 is located lengthwise opposite a recess 30 and overlaps it in height, so that there is a thin vertical wall 32 common to two recesses at approximately half the thickness of the web (Figure 4). This wall is pierced by a round hole 33. The holes 33 are distributed at even intervals along the perforated barrier 27.

The lining 28 consists of a corrugation with horizontal generatrices (device called "in hard way" in reference to the outflow of liquid oxygen) which are not perforated but "serrated". This means that each horizontal or pseudo-horizontal facet of the corrugation is provided with a cut at regular intervals, offset by a quarter of a corrugation step upwards. The width of the cuts, measured along a mantatrice of the corrugation, is of the same order of magnitude as the distance separating each of them from the two adjacent cuts made on the same facet.

The oxygen evaporation passages 18 are open at their upper and lower ends. They contain the corrugation 20 of the lower end up to the level of the webs 22 and are free of any corrugation with respect to the space 29; furthermore, from the upper level of this space 29 up to their upper end, they contain another corrugated spacer 20A analogous to, but not larger than, the corrugation 20. The area of each passage 18 without the corrugation communicates freely with the adjacent space 29 of a passage 19 across a horizontal gap 34 which extends at the same height over the entire length of the exchanger. Every second plate 4 thus extends over the entire length of the exchanger, while every second plate consists of a rectangular plate 4A which extends only up to the height of the web 22 and of a rectangular plate 48 which delimits the distribution chamber 23 for liquid oxygen. The upper surface 35 of the web 22 is inclined laterally in such a way that it slopes from the adjacent plate 4 to the upper edge of the opposite plate 4A. Thanks to a step in the web 22, this surface extends slightly beyond the side of the plate 4A which delimits the passage 18.

In operation, the liquid oxygen bath 5 is maintained at an almost constant level without exceeding the upper side of a vertical plate 5A welded to the exchanger above the windows 26. Thus, the liquid oxygen penetrates laterally into the chambers 23 through one of their ends via the windows 26. At the same time, the gaseous nitrogen below 6 bar absolute into the upper part of the passages 19 via one end of these passages via the space 9 and the distribution corrugations 21.

The liquid oxygen thus forms a liquid column with a practically uniform height above all holes 33. It is pre-distributed over the entire length of the passages 18 in a certain number of jets 36 through these holes 33, then falls freely onto the lining 28, which due to its nature and arrangement ensures a fine distribution of the liquid oxygen over the entire length of the passages 18. The liquid oxygen therefore falls evenly onto the inclined surface 35 of the webs 22, and then flows through the windows 34 into the passages 18.

A film of liquid oxygen thus trickles over the entire metallic surfaces contained in the passages 18, that is to say over the plates 4 and 4A and over the corrugations 20, and partially evaporates in indirect heat exchange with nitrogen during condensation from top to bottom in the alternating passages 19.

As explained above, the passages 18 are not only open at the top and bottom, but are also free of obstacles to the outflow of gaseous oxygen over their entire height. These passages are either smooth (opposite the gap 34) or provided with a simple corrugation 20, 20A with vertical surface lines at every point of their height and have a relatively large pitch. The corrugation 20 improves the heat exchange with the nitrogen through the effect of the wave, while the corrugation 20A serves only as an intermediate piece and can possibly even be partially omitted.

This means that part of the vaporized oxygen can escape at the top of the exchanger and can therefore combine with the vapors from the bath 5 (Figure 1) in the upper dome 7, while the rest of the vaporized oxygen leaves the exchanger at the bottom, together with the excess liquid oxygen, via the line 15. The two exit paths of the vaporized oxygen are connected by a reduced gas flow and each path still causes a minimal pressure loss of this gas outflow. Thus, the height of the exchanger can be increased.

It should be noted that thanks to the structure of the perforated barriers 27, the holes 33 have a horizontal axis and a blind section 37 exists on the back of the barrier, below these holes. Any solid impurities contained in the liquid oxygen can thus settle in these blind sections, which protects the holes 33 from clogging.

Likewise, the arrangement of the vessel 3 in the area of the liquid oxygen bath 5 forms a blind section 38 next to and below the inlet windows 26, which allows the coarsest solid contaminants to settle in this blind section as they leave the feed line 6, as shown at 39 in Figure 2.

If, in a specific application, this offset is considered sufficient to avoid any risk of clogging of the holes 33, it is possible to resort to the variant shown in Figure 5. This differs from the previous one only in the simplified structure of the perforated barrier 27; this is a simple barrier with a rectangular cross-section provided at regular intervals with holes 33 with a vertical axis. These holes can have an increased diameter over most of their height, starting from the bottom, as explained in the above-cited EP-A-0 130 122.

Claims (12)

1. Liquid trickle heat exchanger for evaporating a liquid by heat exchange with a second fluid medium, with a parallelepiped body which is formed by an arrangement of parallel, vertical plates (4) which form between them a plurality of flat passages (18, 19) which are divided into a group of evaporation passages (18) and a group of heating passages (19), each passage in the area of the heat exchange flow comprising a corrugated intermediate piece (20) with vertical surface lines, means for distributing the liquid being provided at the upper end of the exchanger (2) for distributing the liquid over the entire length of the evaporation passages (18), and means (9) for introducing the second fluid medium into the heating passages (19) being provided, characterized in that said means for distributing in chambers (23) are arranged, which are closed at the upper end and are each located above a heating passage (19) from which they are separated by a horizontal web (22), that a horizontal gap (34) extends directly above the web (22) over the entire length of the exchanger and freely connects the lower part of the chamber (23) with an adjacent evaporation passage (18), and that the evaporation passages (18) are open at their two ends at the top and bottom over their entire length and comprise at least one corrugated intermediate piece (20A) with vertical surface lines over their entire length. 2. Heat exchanger according to claim 1, characterized in that the evaporation passages (18) in the region of the gap (34) do not have a corrugated intermediate piece. 3. Heat exchanger according to claim 1 or 2, characterized in that the upper surface (35) of the web (22) is inclined laterally towards the gap (34). 4. Heat exchanger according to at least one of claims 1 to 3, characterized in that the means for distributing the liquid comprise, on the one hand, a horizontal barrier (27) extending over the entire length of each chamber at its mid-height, the thickness of this barrier corresponding to the distance between adjacent plates (4) and comprising openings (33) for distributing the liquid, and, on the other hand, under this barrier, a lining (28) for finely distributing the liquid over the entire horizontal width of the chamber (23). 5. Heat exchanger according to claim 4, characterized in that the said openings (33) form a horizontal row of equally spaced holes. 6. Heat exchanger according to one of claims 4 or 5, characterized in that the barrier (27) has on one vertical surface one or more rear recesses (30) which are closed at the bottom and open at the top, and on its other vertical surface one or more front recesses (31) which are open at the bottom and closed at the top, and that said openings (33) are arranged in a vertical, common wall (32) of the front and rear recesses. 7. Heat exchanger according to claim 6, characterized in that the barrier (27) has a plurality of spaced-apart rear recesses (30) and a plurality of spaced-apart front recesses (31). 8. Heat exchanger according to claim 7, characterized in that the front recesses (31) have a downwardly conically widened shape. 9. Heat exchanger according to at least one of claims 4 to 8, characterized in that the lining (28) is a corrugated intermediate piece with horizontal surface lines, the flanks of which are provided with incisions. 10. Heat exchanger according to at least one of claims 4 to 9, characterized in that the casing (20) is spaced from the upper surface (35) of the web (22). 11. Heat exchanger according to at least one of claims 4 to 10, characterized in that it comprises a lateral liquid inlet space on said chamber (23), the lowest point of this space being located below the lowest point of the inlet window (26) of the chamber. 12. Air separation device comprising a first distillation column (1) operating at a relatively high pressure, a second distillation column operating at a relatively low pressure, and a heat exchanger (2) allowing liquid oxygen from the container of the second column to be brought into thermal exchange with gaseous nitrogen from the top of the first column, characterized in that the heat exchanger is designed according to at least one of claims 1 to 11 and in that the device comprises supply means (6) for supplying liquid oxygen to said means for distributing the liquid and means (9) for supplying gaseous nitrogen to the heating passages.